This invention relates to electrophoretic displays and surfactants that interact with pigments included in electrophoretic media to improve the performance of the media when the media are used in a display. For example, the disclosed surfactants can be added to an electrophoretic medium to improve the brightness of the colored (white) state, the contrast between light (on) and dark (off) states, and the speed of switching between light and dark states for a variety of pigments. The surfactants also diminish images that remain after a display has been switched between two images, a phenomenon known as “ghosting.” The surfactants additionally diminish the amount of unintended switching in the electrophoretic medium in proximity to a pixel, a phenomenon known as “blooming.”
Particle-based electrophoretic displays have been the subject of intense research and development for a number of years. In such displays, a plurality of charged particles (sometimes referred to as pigment particles) move through a fluid under the influence of an electric field. The electric field is typically provided by a conductive film or a transistor, such as a field-effect transistor. Electrophoretic displays have good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Such electrophoretic displays have slower switching speeds than LCD displays, however, and electrophoretic displays are typically too slow to display real-time video. Additionally, the electrophoretic displays can be sluggish at low temperatures because the viscosity of the fluid limits the movement of the electrophoretic particles. Despite these shortcomings, electrophoretic displays can be found in everyday products such as electronic books (e-readers), mobile phones and mobile phone covers, smart cards, signs, watches, shelf labels, and flash drives.
An electrophoretic image display (EPID) typically comprises a pair of spaced-apart plate-like electrodes. At least one of the electrode plates, typically on the viewing side, is transparent. An electrophoretic fluid composed of a dielectric solvent with charged pigment particles dispersed therein is enclosed between the two electrode plates. An electrophoretic fluid may have one type of charged pigment particles dispersed in a solvent or solvent mixture of a contrasting color. In this case, when a voltage difference is imposed between the two electrode plates, the pigment particles migrate by attraction to the plate of polarity opposite that of the pigment particles. Thus, the color showing at the transparent plate can be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate to the opposite plate, thereby reversing the color. Alternatively, an electrophoretic fluid may have two types of pigment particles of contrasting colors and carrying opposite charges and the two types of pigment particles are dispersed in a clear solvent or solvent mixture. In this case, when a voltage difference is imposed between the two electrode plates, the two types of pigment particles would move to opposite ends (top or bottom) in a display cell. Thus, one of the colors of the two types of the pigment particles would be seen at the viewing side of the display cell.
Many commercial electrophoretic media essentially display only two colors, with a gradient between the black and white extremes, known as “grayscale.” Such electrophoretic media either use a single type of electrophoretic particle having a first color in a colored fluid having a second, different color (in which case, the first color is displayed when the particles lie adjacent the viewing surface of the display and the second color is displayed when the particles are spaced from the viewing surface), or first and second types of electrophoretic particles having differing first and second colors in an uncolored fluid. In the latter case, the first color is displayed when the first type of particles lie adjacent the viewing surface of the display and the second color is displayed when the second type of particles lie adjacent the viewing surface). Typically the two colors are black and white.
If a full color display is desired, a color filter array may be deposited over the viewing surface of the monochrome (black and white) display. Displays with color filter arrays rely on area sharing and color blending to create color stimuli. The available display area is shared between three or four primary colors such as red/green/blue (RGB) or red/green/blue/white (RGBW), and the filters can be arranged in one-dimensional (stripe) or two-dimensional (2×2) repeat patterns. Other choices of primary colors or more than three primaries are also known in the art. The three (in the case of RGB displays) or four (in the case of RGBW displays) sub-pixels are chosen small enough so that at the intended viewing distance they visually blend together to a single pixel with a uniform color stimulus (‘color blending’). The inherent disadvantage of area sharing is that the colorants are always present, and colors can only be modulated by switching the corresponding pixels of the underlying monochrome display to white or black (switching the corresponding primary colors on or off). For example, in an ideal RGBW display, each of the red, green, blue and white primaries occupy one fourth of the display area (one sub-pixel out of four), with the white sub-pixel being as bright as the underlying monochrome display white, and each of the colored sub-pixels being no lighter than one third of the monochrome display white. The brightness of the white color shown by the display as a whole cannot be more than one half of the brightness of the white sub-pixel (white areas of the display are produced by displaying the one white sub-pixel out of each four, plus each colored sub-pixel in its colored form being equivalent to one third of a white sub-pixel, so the three colored sub-pixels combined contribute no more than the one white sub-pixel). The brightness and saturation of colors is lowered by area-sharing with color pixels switched to black. Area sharing is especially problematic when mixing yellow because it is lighter than any other color of equal brightness, and saturated yellow is almost as bright as white. Switching the blue pixels (one fourth of the display area) to black makes the yellow too dark.
Surfactants have been used for some time with electrophoretic media to improve the performance of the media. For example, U.S. Patent Publication No. 2002/0180687 describes the use of polyhydroxy and aminoalcohol surfactants to stabilize charged electrophoretic particles dispersed in a non-polar fluid. In particular, polyhydroxy surfactants were found to reduce particle flocculation and to prevent electrophoretic particles from attaching to the walls of the capsules containing the electrophoretic media. Other references, such as U.S. Pat. No. 7,405,865, suggest that the contrast between light and dark states can be improved with the inclusion of alkyne surfactants including acetylene glycol derivatives. Such surfactants can also improve the durability and shelf-life of electrophoretic media by decreasing the likelihood that the electrophoretic particles will settle out from the fluid of the electrophoretic medium.
Nonetheless, although seemingly simple, electrophoretic media and electrophoretic devices display complex behaviors. For instance, it has been discovered that simple “on/off” voltage pulses are insufficient to achieve high-quality text in electronic readers. Rather, complicated driving schemes (waveforms) are needed to drive the particles between states and to assure that the new displayed images do not retain a memory of the previous image, i.e., a “ghost.” See, for example, U.S. Patent Publication No. 20150213765. Compounded with the complexities of the electric fields, the internal phase, i.e., the mixture of particles (pigment) and fluid, can exhibit unexpected behavior due to interactions between charged species and the surrounding environment (such as an encapsulation medium, e.g., a coacervate or polymer microcell) upon the application of an electric field. Additionally, unexpected behaviors may result from impurities in the electrophoretic medium or encapsulation medium. Accordingly, it is difficult to predict how an electrophoretic display will respond to variations in the internal phase composition. In many cases, optimizing image quality is both a function of the size and shape of the waveform, the components of the electrophoretic medium, and the components of the encapsulation medium.